Perturbations in hormone levels, growth, body habitus, and behavior are generally believed to be related during human puberty. A central issue is whether increases in hormone levels directly influence behavior or whether the adolescent's perception of his/her secondary sex characteristics provides the major basis for behavioral change. The present proposal will attempt to determine the direct influence of sex hormones on behavior by performing sequential behavior testing in 55 sex-steroid-deficient adolescents before, during and after they receive therapeutic hormone replacement therapy. Boys and girls with primary or secondary hypogonadism are receiving three dose levels of intramuscular testosterone enanthate (boys) or oral conjugated estrogen (girls) alternating with placebo over 3-month periods. The study design is randomized, double-blind, placebo controlled cross-over, clinical trial. To assess longer term behavioral effects of sex steroids, subjects will be retested 12-18 months after completion of the initial 21 month protocol.